CFA Level I Exam · The Firm and Market Structures

Perfect Competition: Short-Run and Long-Run Equilibrium Explained

Updated 7 October 2026 · Fact-checked

Perfect competition is a market with many small sellers, identical products, free entry and exit, and price-taking firms. To solve questions, produce where marginal revenue (equal to price) equals marginal cost, then compare price with average total cost and average variable cost to decide whether to operate, break even or shut down.

Understand Perfect Competition

In perfect competition, many buyers and sellers trade an identical product. Each firm is tiny relative to the market. Information is freely available and entry and exit are free. No single firm can move the market price.

This makes each firm a price taker. The market sets the price through supply and demand. The firm's demand curve is perfectly elastic (horizontal) at that price. So the firm's marginal revenue (MR) equals the price (P), and also equals average revenue (AR). Whether it sells one more unit or a hundred more, each unit brings in P.

The firm maximizes profit by producing where MR = MC, with marginal cost rising. Producing a unit that costs less than P adds profit. Producing a unit that costs more than P reduces profit. Output stops at the point where the next unit's marginal cost reaches the price. Profit per unit at that output is P − ATC.

In the short run, the number of firms is fixed. A firm can earn economic profit (P > ATC), a loss (P < ATC), or normal profit (P = ATC). If P is below ATC but at or above AVC, the firm keeps producing because it covers variable costs and part of fixed costs. If P falls below AVC, it shuts down and loses only its fixed costs. The firm's short-run supply curve is its MC curve above minimum AVC.

In the long run, all costs are variable and firms can enter or leave. Economic profits attract entrants, supply rises and price falls. Losses push firms out, supply falls and price rises. The process ends when P = minimum ATC and economic profit is zero. At that point firms produce at efficient scale, and the market is allocatively and productively efficient. Shifts in demand or cost conditions move the market price and trigger the adjustment again.

Key formulas to remember

Price-taker revenue: MR = AR = P; True for a perfectly competitive firm. Total revenue = P × Q.
Profit-maximizing output: MR = MC (with MC rising), so P = MC; Produce this quantity if the shutdown rule is satisfied.
Economic profit: Profit = (P − ATC) × Q = TR − TC; Positive profit attracts entry. Negative profit drives exit in the long run.
Short-run shutdown rule: Shut down if P < AVC; operate if P ≥ AVC; At P = AVC the firm is indifferent, since either choice loses total fixed cost.
Breakeven point: P = minimum ATC (economic profit = 0); Long-run equilibrium price in perfect competition.
Long-run equilibrium: P = MR = MC = minimum ATC; Zero economic profit and no entry or exit.
Total cost link: ATC = AFC + AVC; Use this to move between average cost measures.

How to solve Perfect Competition questions

Use the same sequence for any perfect competition question, whether it gives a table, a cost function or a description.

1Confirm the structure: many sellers, identical product, free entry and exit. Then set MR = AR = P.
2Find the output where MC = MR (= P). If you have a table, pick the highest quantity where MC ≤ P.
3Calculate total revenue (P × Q) and total cost at that output, or P − ATC per unit.
4Compare P with ATC: above means economic profit, equal means breakeven, below means loss.
5If P < ATC, compare P with AVC. If P ≥ AVC, operate in the short run. If P < AVC, shut down.
6For long-run questions, ask what entry or exit does to supply and price until P = minimum ATC and economic profit is zero.
7Check the answer: does the conclusion match the stated time frame (short or long run)?

Quickest way: Three-line price comparison

When to use it: Use this when the question gives P, ATC and AVC and asks what the firm does or what happens next.

Compare P with ATC. If P > ATC, profit: expect entry in the long run and falling price.
If P < ATC, compare with AVC. P ≥ AVC: operate and take the smaller loss. P < AVC: shut down.
If P = minimum ATC, it is long-run equilibrium: zero economic profit and no entry or exit.
Eliminate options that say a firm shuts down when P is above AVC, or that sustained economic profit persists in the long run.

Common mistakes in Perfect Competition

Using ATC instead of AVC for the shutdown decision
Students see a loss and assume the firm should stop producing.
Fix: Shutdown depends on P versus AVC in the short run. A loss with P ≥ AVC means keep operating because fixed costs are sunk.
Saying a firm earns zero profit means it makes no money in an accounting sense
Economic profit and accounting profit are confused.
Fix: Zero economic profit means revenue covers all costs, including the opportunity cost of capital (normal profit).
Setting output where P = ATC instead of P = MC
Breakeven and profit maximization get mixed up.
Fix: Choose output with MR = MC first. Then use ATC at that output to find profit or loss.
Picking an output where MC equals P on the falling part of the MC curve
MC = MR can occur twice on a U-shaped MC curve.
Fix: Choose the point where MC is rising and cuts MR from below.
Expecting long-run economic profit in a perfectly competitive market
Short-run profit is treated as permanent.
Fix: Free entry competes profits away. Long-run economic profit is zero, with P = minimum ATC.
Thinking an individual firm can change the market price by changing output
Students apply monopoly logic.
Fix: A price taker faces a horizontal demand curve. Only market supply and demand shifts change price.

Worked examples

Example 1

A perfectly competitive firm faces a market price of $20. Its marginal cost is $14 at 100 units, $18 at 120 units, $20 at 140 units and $24 at 160 units. At 140 units, ATC is $17 and AVC is $12. What is the firm's economic profit at the profit-maximizing output? A) $420, B) $1,120, C) $2,380.

Show the solution

MR = P = $20. MC = $20 at 140 units, so the firm produces 140 units.
Profit per unit = P − ATC = 20 − 17 = $3.
Economic profit = 3 × 140 = $420.
Check the other options: $1,120 is (P − AVC) × Q = (20 − 12) × 140, which ignores fixed cost. $2,380 is total cost (17 × 140), not profit.
Long run: positive economic profit attracts entrants. Supply rises and price falls until economic profit is zero.

Answer: Economic profit is $420 (option A). In the long run, entry pushes price down toward minimum ATC.

Example 2

In the short run, a perfectly competitive firm faces a price of €9. At its MR = MC output of 500 units, ATC is €11 and AVC is €8. What should the firm do, and what is its loss? A) operate, loss €500; B) operate, loss €1,000; C) shut down, loss €1,500.

Show the solution

Compare P with ATC: 9 < 11, so the firm makes an economic loss.
Compare P with AVC: 9 > 8, so revenue covers variable costs and the firm should operate in the short run.
Loss if operating = (ATC − P) × Q = (11 − 9) × 500 = €1,000.
Check against shutdown: AFC = ATC − AVC = 11 − 8 = €3, so total fixed cost = 3 × 500 = €1,500. Shutting down loses €1,500, which is more than the €1,000 lost by operating.
Check the other options: €500 is (P − AVC) × Q = (9 − 8) × 500, the contribution toward fixed cost, not the loss.

Answer: Operate, with a loss of €1,000 (option B). This is less than the €1,500 loss from shutting down.

Exam tips

Questions often hand you P, ATC and AVC and ask what the firm does. Compare P with ATC first, then AVC.
Watch the time frame. Short-run answers allow profit or loss. Long-run answers settle at zero economic profit.
Do not let an unlisted fixed cost distract you. Fixed cost matters only through ATC and the size of the loss.
Remember the supply curve fact: the short-run firm supply curve is MC above minimum AVC.
With three options and no penalty for wrong answers, eliminate any option claiming long-run economic profit or shutdown when P ≥ AVC.

Practice questions from The Firm and Market Structures

Perfect Competition in other exams

The same ground in other exams, if you are preparing for more than one or want another angle on it.

Perfect Competition: frequently asked questions

What is the difference between the shutdown point and the breakeven point?

The shutdown point is where price equals minimum AVC. Below it, the firm stops producing in the short run. The breakeven point is where price equals minimum ATC, so economic profit is zero. Between the two, the firm makes a loss but keeps operating.

How do I find the profit-maximizing output in perfect competition?

Set price equal to marginal cost, since MR equals P for a price taker. Choose the output where MC is rising and equals P. Then check that P covers AVC, otherwise the firm shuts down.

Why is long-run economic profit zero in perfect competition?

Free entry lets new firms enter whenever profit exists. Extra supply lowers the price until P equals minimum ATC. Losses cause exit, which raises price back to the same point.

Does a firm always stop producing when it makes a loss?

No. In the short run, it keeps producing if price covers average variable cost, because it then loses less than its fixed costs. It only shuts down when price is below AVC.